LT1769IGN datasheet(13/16 Pages) LINER

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LT1769

APPLICATIONS INFORMATION

Figure 7. Lower VBOOST

For example, VX = 3.3V then:

Then PDRIVER =

()(

)( ) +









()

BAT

Total Power in the IC is: 0.35 + 0.43 + 0.5 = 1.3W

Temperature rise will be (1.3W)(35

°C/W) = 46°C. This

assumes that the LT1769 is properly heat sunk by con-

necting the eleven fused ground pins to expanded traces

and that the PC board has a backside or internal plane for

heat spreading.

The PDRIVER term can be reduced by connecting the boost

diode D2 (see Figure 7) to a lower system voltage (lower

than VBAT) instead of VBAT.

BOOST

SPIN

1769 F07

LT1769

IVX

µF

DRIVER =

()( )( ) +









()

126

55 19

009

The average IVX required is:

DRIVER

009

The previous example shows the dramatic drop in driver

power dissipation when the boost diode (D2) is connected

to an external 3.3V source instead of the 12.6V battery.

PDRIVER drops from 0.43W to 0.09W resulting in an

approximately 12

°C drop in junction temperature.

Fused-lead packages conduct most of their heat out the

leads. This makes it very important to provide as much PC

board copper around the leads as is practical. Total

thermal resistance of the package-board combination is

dominated by the characteristics of the board in the

immediate area of the package. This means both lateral

thermal resistance across the board and vertical thermal

resistance through the board to other copper layers. Each

layer acts as a thermal heat spreader that increases the

heat sinking effectiveness of extended areas of the board.

Total board area becomes an important factor when the

area of the board drops below about 20 square inches. The

graph in Figure 8 shows thermal resistance vs board area

for 2-layer and 4-layer boards with continuous copper

planes. Note that 4-layer boards have significantly lower

thermal resistance, but both types show a rapid increase

for reduced board areas. Figure 9 shows actual measured

lead temperatures for chargers operating at full current.

3.5mA V

1.5mA V

7.5mA

0.012 I

55 V

BIAS

BAT

DRIVER

BAT

()( )+

()

+ ()

()( )

[]

()(

) +









()

= () (

)(

)

()( )( )( )

VBAT

3.5mA 19

1.5mA 12.6

12.6

7.5mA

0.012 2000mA

0.35W

2 12.6

55 19

0.43W

0.16 12.6

19 2 200kHz

0.42 0.08

0.5W

BIAS

DRIVER

()( )+

()

+ ()

()(

)

[]=

()( ) +









()

= () (

)( )

()( )(

)

−

12 6

RSW = Switch ON resistance ≈ 0.16Ω

tOL = Effective switch overlap time ≈ 10ns

f = 200kHz

Example: VIN = 19V, VBAT = 12.6V, IBAT = 2A:

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